X-ray imaging devices generally employ a collimated beam of radiation which is directed from a focal spot through an object to be scanned, such as a live human or animal patient, or a package to be inspected. A bank of radiation-sensitive detectors is located opposite the focal spot, with the object to be scanned between the focal spot and the detectors. The focal spot and detectors may be fixed relative to one another on a gantry which rotates about the object to be scanned. The detectors receive radiation which has passed through the object and which has therefore been attenuated to varying degrees as a function of the density of structures within the object and in the radiation path. The detectors generate signals which correspond to the detected density values, and these signals are used to map the object so that the internal structures can be seen.
The radiation beam is collimated by passage of the beam through a slit in a radiation-opaque plate. Typically the plate contains a number of slits of differing widths, so that the beam can be collimated to different widths. For convenience, the slit plate is generally moved into the desired position by translation in a direction transverse to the direction of the beam (generally referred to as the z-direction).
It is important that the slit through which the radiation beam passes to be collimated be located properly relative to the focal spot and the detectors so as to admit precisely as much radiation as can be detected by the detectors, for maximum data acquisition for each scan of the object. If the slit is not properly aligned with the focal spot and the detectors, some of the radiation pass through the object but may not be received by the detectors. Any radiation which passes through the object without being detected subjects the object to radiation exposure without providing useful imaging data, and this is undesirable.
It is also important that movement of the slit plate be smooth, without backlash, accurate and precise. This is especially critical in scanners which may rotate at speeds greater than one revolution per second.
In prior art scanners, the slit plate is typically mounted on a slide which is adapted for travel along a set of parallel shafts. The slide is driven by a drive mechanism, such as a precision leadscrew, which is mounted to a stepper motor. Misalignment of the shafts, of the slide on the shafts, and binding of the leadscrew, commonly occur because it is difficult to maintain these structures in precise alignment. Because each structure is independently mounted, alignment of all three is a costly and time-consuming undertaking and must be checked and repeated frequently.
It would therefore be an advancement in the art of precision drive assemblies to overcome the disadvantages of the prior art.